Someone in a multiplayer video game who waits for someone to weaken their enemy and kills them last second so instead of the person who did all the work getting credit the the person who stole gets it.
Person 1: WOW, that's so gay!
Person B: What happened?
Person 1: This noob just stole my kill!
Person B: I fuckin hate Kill Stealers!
One who interrupts a fight and delivers the final blow on a target that someone else has been wearing down, taking all the credit. Primarily used in multiplayer videogames, especially the Super Smash Bros. series from which it was born.